Imaging gas for improved resolution in imaging chamber of electron radiography system

ABSTRACT

An electron radiography system with improved resolution obtained by incorporating a few per cent of an electro-negative gas with the imaging gas to combine with the electrons forming negative ions for attracting to the anode and depositing on the dielectric sheet.

United States Patent [191 Proudian et a1.

[4 Aug. 20, 1974 IMAGING GAS FOR IMPROVED RESOLUTION 1N iMAGING CHAMBER OF ELECTRON RADIOGRAPHY SYSTEM [75] Inventors: Andrew P. Proudian, Chatsworth;

Paul B. Scott, Topanga, both of Calif.

[73] Assignee: Xonics, Inc., Van Nuys, Calif.

[22] Filed: Sept. 28, 1973 [21] Appl. No.: 401,689

[52] US. Cl .(LT250/5i5, 2507375, 250/383 [51] Int. Cl. G03b 41/16 [58] Field of Search 250/315, 315 A, 374, 375,

[56] References Cited UNITED STATES PATENTS 2,922,911 1/1960 Friedman 313/93 H/6H VOLT/76E SUPPL Y 2,936,388 5/1960 Chubb et a1. 313/93 3,526,767 9/1970 Roth et a1. 250/315 3,774,029 11/1973 Muntz et a1 250/315 Primary Examiner-William F. Lindquist Attorney, Agent, or Firm1-1arris, Kern, Wallen & Tinsley [5 7] ABSTRACT An electron radiography system with improved resolution obtained by incorporating a few per cent of an electro-negative gas with the imaging gas to combine with the electrons forming negative ions for attracting to the anode and depositing on the dielectric sheet.

12 Claims, 1 Drawing Figure This invention relates to the creation of X-ray images without the use of conventional X-ray film, sometimes referred to as electron radiography, such as is described in the copending application of Muntz et al., Ser. No. 261,927, filed June 12, 1972, entitled RADIO- GRAPl-IIC SYSTEMS WITH XEROGRAPHIC PRINTING, and assigned to the same assignee as the present application. In an electron radiography system, an X-ray opaque gas at high pressure is maintained in a gap between electrodes in an imaging chamber to produce electrons within the gap as a function of X- rays entering the gap. The electrons are collected on a dielectric sheet placed at the anode, resulting in a latent electrostatic charge on the sheet. The latent image is then made visible by xerographic techniques.

In order to improve absorption efficiency of the imaging chamber, the gas in the chamber may be maintained at superatmospheric pressure, and the thickness of the X-ray absorbing gas layer may be made as large as two centimeters or more. When such thicknesses of absorbing gas layer are used, the electrostatic image formed in the imaging chamber can be blurred or unsharp and exhibit poor resolution, due to two effects which are dependent on gap thickness.

The first effect is that of geometric unsharpness due to oblique X-rays, and is handled in a manner not related to the present invention. Z

A second significant source of geometric unsharpness is the diffusive spreading of the charges as they travel from their point of creation to the insulating receptor. Proper orientation of the collective electric fields cannot overcome this problem. The purpose of the present invention is to reduce image blurring due to diffusion and to improve the resolution of the imaging chamber of the electron radiographic system. In order to clarify the basis of the solution proposed in the present invention, it is ncessary to describe the process of charge creation by the X-rays and their transport to the dielectric receptor in some detail.

In the present art, the gas in the imaging chamber consists of a high Z radiopaque gas such as Xenon or Krypton, together possibly with a small amount of a quenching" gas such as methane, which is known experimentally to suppress electrical breakdown sparks in gas discharges.

When an X-ray photon incident upon a Xenon atom is absorbed by the latter, it emits an electron, typically from its most tightly bound shell or K-shell, and typically with an energy of ten to a few tens of kilovolts. This ejected photoelectron rather rapidly dissipates its energy by further ionization of Xenon atoms with which it collides, leading to a ball or cloud of ionization around the initial absorbing atom, consisting of typically a thousand or so electron/Xenon-ion pairs, the electrons having energies typically less than the ionization energy of Xenon, and the Xenon ions being in various excited states. The diameter of this ball is typically X to 10 cm at the typical operating pressures.

If there were no electric field in the gas gap due to the voltage difference impressed between the electrodes of the imaging chamber, the cloud electronswould eventually be thermalized (i.e., achieve a mean thermal kinetic energy (3/2)k'l", T gas temperature) and would recombine with the Xenon ions, leading to eventual complete neutralization of the gas, although such recombination is slow in gas kinetic terms for a process such as Xe +e' Xe.

In the presence of the accelerating electric field due to the applied voltage difference, two things happen: On the one hand, the electrons drift under the influence of the electric field 'E in the direction of while the ions drift in the direction of E. On the other hand, the electrons gain energy from the applied field by the combined process of acceleration by the field and inefficient collisional energy exchange with the chamber gas atoms. By contrast, the ions, mostly because of their far greater mass (of the order of 200,000 electron masses), transfer energy efficiently with the gas atoms, and remain essentially at the (room) temperature of these neutral atoms (which constitute all but 1 part in about 10 of the gas).

The electrons pick up significant energy from the field and achieve energies of several electron volts, compared to 1/40 of an electron volt thermal energy corresponding to the gas and positive ion temperature at 300K (nominal).

The electrons and ions respectively drift towards the anode and cathode under the influence of the applied field, and, at the same time, they both undergo random thermal motions due to collisions with neutral atoms, these motions being the underlying source of diffusion.

' The electron motions, due to their small mass and large energy, are several thousand times as fast as the ions, and severely diffuse the resulting electron image deposited on the receptor.

The ions, on the other hand, would on their own produce an image much less blurred due to their own diffusion. The characteristic spot size D due to diffusion for electrons and ions created a distance d from the respective collecting insulator is, approximately where E is the thermal energy of the charge carrier (electron or ion), V is the accelerating voltage difference and q electron charge (=1.6 X l0 C). Since E for electrons is z 5 to 10 electron volts, where E for ions is z l/40 electron volt, itis clear that for typical values d 1 cm and V 10,000 volts, we have D z .3 mm (corresponding to a 3 lp/mm resolution limit), while for ions D z .015 mm (corresponding to lp/mm resolution limit due to diffusion).

One might conclude from the above that positive charge images will have much higher resolution than negative charge images, or, in other words, that intercepting the charges headed towards the cathode with an insulating receptor will result in a higher resolution latent image (other factors being equal).

This is not quite the case, however, because while the positive ions themselves diffuse negligibly, they suffer from the diffusiveeffects of the electrons, because of the recombination/ionization processes in the gas: indeed, the positive ions which reach the image receptor at the cathode are not just those created in the initial ball of ionization. Some of those are neutralized by electron/ion recombinatiomwhile others'are created by electron-atom collision, and the latter are of course created everywhere along the path of the random motion of the electrons, so that their spatial distribution reflects the random diffusive motions of the electrons.

The effective ion diffusion thermal energy (E of the ions lies therefore actually somewhere between the pure electron and pure ion values, the actual value depending on the ratio of one ionization/recombination cycle time (which leads to no net ionization gain) to the drift time of the positive ions. Experimental data indicate that the effective ion thermal diffusion energy E is of the order of half an electron volt, that is perhaps or times that of the pure-ion case, when the chamber is operated at approximately unit gain (no excess electron/ion pairs collected beyond those created in the primary absorption process).

Of course, if the chamber is operated with net gain, then most of the ions (or electrons) collected must have been created by secondary ionizing electron collisions, and the diffusive spread will then be characteristic of electron transport, with a large characteristic diffusive spot size. But the point is that even at a net gain of unity, electron transport will partly affect positive ion diffusive spread because some of the collected ions will have been created by collision processes with the highly diffusive electrons.

The above problem cannot be alleviated by reducing the electric field in the gas to values so low that the electrons do not gain sufficient energy from the field to cause secondary ionization. At such field values, and corresponding electron energies, a large fraction of the electrons will recombine (without corresponding reionization), and the total collected charge will be far reduced, leading to unacceptable loss of system sensitivity.

The solution to the above problem lies in simultaneously inhibiting both electron diffusion and the recombination/ionization process, so that there is neither loss of sensitivity on the one hand, nor loss of resolution on the other.

The is achieved in the present invention by replacing the electrons as carriers of the negative charge by a heavy ion (and therefore non-diffusing), nonrecombining negative charge carrier. The source of the carrier ion is an electronegative gas added to the imaging gas. An excellent choice for such a carrier is the negative ion SP of sulfur hexafluoride: sulfur hexafluoride (SP is an extremely active electrophyllic molecule, which attaches electrons by very rapidattachment processes (e.g., SP e third body SP third body), and with a binding energy of approximately 4 electron volts.

Thus, in the system of the present invention, the working gas, rather than being a pure high Z gas such as Xenon, or such a gas with a small fraction of quenchant gas, also contains a few per cent, typically 2 to 10%, of an electronegative attaching gas with a strong affinity, such as SP As a result, the electrons created in the ionization cloud by the primary electron are rapidly captured by the attaching gas, such as SF forming a stable negative ion, such as SP The latter will then drift towards the collecting anode or receptor (while the positive ions drift toward the cathode), but will neither create new ions nor recombine in any significant amount during the drift time between creation by attachment and collection, because recombination processes of the form SP Xe" SF Xe between a highly stable negative ion such as SP and a relatively weakly electrophyllic ion such as Xe are quite slow.

. cent of an electronegative 4 Nor will the heavy ions diffuse significantly at all, since their thermal kinetic energy will remain at or close to room temperature. Thus, by introduction of a small fraction of a strong electron attacher such as SP the system resolution at large gaps can be considerably improved.

Accordingly, it is an object of the present invention to provide a new and improved apparatus and method for improving the resolution in electron radiographic systems by incorporating a few per cent of an electronegative gas into the imaging gas. Other objects, advantages, features and results will more fully appear in the course of the following description. The single FIG- URE of the drawing illustrates a typical electron radiographic system with imaging chamber and incorporating the presently preferred embodiment of the invention.

X-rays are directed from a source 10 past the object 11 being X-rayed to the imaging chamber 12, which may be conventional in design such as set out in the aforementioned copending application. A typical imaging chamber includes a housing 13 carrying a cathode 14 on an insulator 15, with an anode 16 carried on a housing cover 17. Alternatively, the housing cover may serve as the anode. The dielectric sheetreceptor 18 is carried on the anode, with the imaging gas introduced into the chamber at 19 filling the gap between the electrodes. An electric field is produced across the gap by a high voltage supply 22 connected to the electrodes.

The imaging gas is a high Z, X-ray opaque gas such as Xenon or Krypton maintained at high pressure, typically 5 to 20 atmospheres. A small amount of a quenchant such as methane may be added but is not normally required. in a system of the invention, a few per gas is also added, typically in the order of 2 to 10%.

The system is operated in the conventional manner in making the X-ray exposure. The dielectric sheet is mounted on the anode and the chamber is sealed. The gas is introduced into the chamber at the pressure desired and the high voltage supply is turned on. The X-ray source is turned on to make the exposure, producing the electrostatic charge image on the receptor. The high voltage supply is turned off, the pressure in the chamber is reduced, the chamber is opened and the receptor sheet is removed for developing and fixing of the visual image.

An electronegative gas has a lower energy when an electron is added to the molecule forming a negative ion. A preferred electronegative gas has a strong affinity for electrons, a high attachment rate for electrons, and forms a heavy negative ion which is nonrecombining in nature. These various characteristics depend upon the molecular structure of the electronegative gas and can be determined by testing of gases. The halogen compounds are suitable for use as sources of the negative ions. The following gases are suitable: sulfur hexafluoride, carbon tetrachloride, tungsten hexafluoride, uranium hexafluoride, oxygen and perfluorohexene.

The following test results illustrate the improvement in resolution obtained. Resolution is measured in line pairs per millimeter for various gases in the gap. The X-ray source potential is measured in kilovolts peak. In Table l, the gap in the imaging chamber was 5 mm and the pressure was 5 atmospheres (absolute). In Table II,

the gap was 6 mm and the pressure 5 atmospheres (absolute) while in Table Ill, the gap was 12.6 mm and the pressure 8 atmospheres (absolute).

Source Xenon With With With (kvp) Only l% SF 5% SF 5% Air Source Xenon With With (kvp) Only 4% SF 2% CCl Ill Source Xenon With With With (kvp) Only 2% SF 2% O 4%0 We claim:

1. In a radiographic system for operation with a source of X-rays and having a pair of electrodes comprising an anode and a cathode,

first means for supporting said electrodes in spaced relation with a small gap therebetween and for maintaining a superatmospheric pressure in said gap with a dielectric sheet in said gap at said anode,

an X-ray absorber and electron and positive ion emitter in said gap between said anode and cathode for producing a charge image on' said dielectric sheet, said emitter comprising an X-ray opaque gas at superatmospheric pressure and having an atomic number of at least 36, and

second means for connecting a high voltage electric power supply across said electrodes for attracting negatively charged particles toward said anode for deposit of said charged particles on said dielectric sheet,

the improvement comprising including in said X-ray opaque gas a few percent of an electronegative gas for combining with electrons in the gap forming negative ions for attraction to said anode.

2. A system as defined in claim 1 wherein said electronegative gas is an electronegative halogen compound.

3. A system as defined in claim 1 wherein said electronegative gas is selected from the group consisting of positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged;

passing X-rays through said object and one of said electrodes;

absorbing incoming X-rays in the gap by maintaining in the gap an X-ray opaque gas of atomic number at least 36 at superatomspheric pressure and generating electrons and positive ions in the gas; converting electrons in the gap to negative ions by combining electrons with electronegative gas molecules maintained in the gap with said X-ray opaque gas; and attracting negatively charged particles toward the anode by applying a high potential across the elec trodes depositing said charged particles on the dielectric sheet.

8. A method as defined in claim 7 wherein said electronegative gas is an electronegative halogen compound.

9. A method as defined in claim 7 wherein said electronegative gas is selected from the group consisting of sulfur hexafluoride, carbon tetrachloride, tungsten hexafluoride, uranium hexafluoride, oxygen and perfluorohexene.

10. A method as defined in claim 7 wherein said electronegative gas is sulfur hexafluoride. I

11. A method as defined in claim 7 wherein said electronegative gas is carbon tetrachloride.

12. A method as defined in claim 7 wherein said electronegative gas is oxygen. 

1. In a radiographic system for operation with a source of Xrays and having a pair Of electrodes comprising an anode and a cathode, first means for supporting said electrodes in spaced relation with a small gap therebetween and for maintaining a superatmospheric pressure in said gap with a dielectric sheet in said gap at said anode, an X-ray absorber and electron and positive ion emitter in said gap between said anode and cathode for producing a charge image on said dielectric sheet, said emitter comprising an X-ray opaque gas at superatmospheric pressure and having an atomic number of at least 36, and second means for connecting a high voltage electric power supply across said electrodes for attracting negatively charged particles toward said anode for deposit of said charged particles on said dielectric sheet, the improvement comprising including in said X-ray opaque gas a few percent of an electronegative gas for combining with electrons in the gap forming negative ions for attraction to said anode.
 2. A system as defined in claim 1 wherein said electronegative gas is an electronegative halogen compound.
 3. A system as defined in claim 1 wherein said electronegative gas is selected from the group consisting of sulfur hexafluoride, carbon tetrachloride, tungsten hexafluoride, uranium hexafluoride, oxygen and perfluorohexene.
 4. A system as defined in claim 1 wherein said electronegative gas is sulfur hexafluoride.
 5. A system as defined in claim 1 wherein said electronegative gas is carbon tetrachloride.
 6. A system as defined in claim 1 wherein said electronegative gas is oxygen.
 7. A method of producing an electrostatic image on a dielectric sheet, including the steps of: positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged; passing X-rays through said object and one of said electrodes; absorbing incoming X-rays in the gap by maintaining in the gap an X-ray opaque gas of atomic number at least 36 at superatomspheric pressure and generating electrons and positive ions in the gas; converting electrons in the gap to negative ions by combining electrons with electronegative gas molecules maintained in the gap with said X-ray opaque gas; and attracting negatively charged particles toward the anode by applying a high potential across the electrodes depositing said charged particles on the dielectric sheet.
 8. A method as defined in claim 7 wherein said electronegative gas is an electronegative halogen compound.
 9. A method as defined in claim 7 wherein said electronegative gas is selected from the group consisting of sulfur hexafluoride, carbon tetrachloride, tungsten hexafluoride, uranium hexafluoride, oxygen and perfluorohexene.
 10. A method as defined in claim 7 wherein said electronegative gas is sulfur hexafluoride.
 11. A method as defined in claim 7 wherein said electronegative gas is carbon tetrachloride.
 12. A method as defined in claim 7 wherein said electronegative gas is oxygen. 